Optional Practical: Falling cotton reel


In this practical a cotton reel will be dropped down a guide rod onto a spring as shown in the photo. When the reel hits the spring it compresses it then bounces back up again. If the spring is completely compresses you will hear it hit the table with a small bang. If not completely compressed then the noise is slightly less (boing). Try dropping the reel from different heights to see if you can hear the difference.


When the reel falls its PE is converted into KE and when it compresses the spring KE will then be converted into elastic PE. If energy is conserved then

mgh = ½kx2


m = mass of reel
k= spring constant
h= Height fallen
x= compression of the spring


If you increase the height of release until you just hear a bang then you know that the spring has been fully compressed. Change the mass of the reel by adding lengths of solder (soft wire made mainly out of lead) then find the minimum height of release necessary to completely compress the spring (make a bang). Use a graphical method to verify mgh = ½kx2 and find a value for the spring constant k.

Gravitational to elastic PE 

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